We have found bacterial mutants that are highly sensitive to visible and near-ultraviolet light and to a tryptophan photoproduct. These mutants also lack the ability to undergo genetic recombination (i.e., they are recombinationless mutants). In addition to this lethal effect, mutations are also produced by these agents. The broad objectives of this proposal are: (1) to separate and identify the tryptophan photoproduct that causes these biological damages. Also, to separate and identify any other photo-receptors (chromophore) that lead to alterations in the DNA of bacterial cell; (2) to identify the specific molecular alterations that account for mutation by light (above 300 nm) and by tryptophan photoproduct (i.e., base pair change, transition, transversion, frameshift, deletion); and (3) to identify the macromolecule with which tryptophan photoproduct interacts (i.e., DNA, or one of the repair enzymes, etc.).